A. Field of the Invention
The present invention relates to portable antennas for transmitting and receiving ultra-high frequency radio signals. More particularly, the invention relates to a compact, manpack-portable, foldable multi-band antenna which has an integral diplexer that eliminates the requirements for transporting heavy, bulky external diplexer boxes along with the antenna when an operator in the field needs to communicate in different frequency bands with a satellite.
B. Description of Background Art
Governmental agencies such as U.S. military services that utilize personnel operating in remote field locations have a need for instantaneous, reliable communication systems. Such systems are required for conveying data between personnel in field locations and fixed command and control sites. As a practical matter, communication systems which meet the various requirements for reliable communications of the type alluded to above generally utilize radio transceivers. Thus the U.S. military services and other governmental agencies typically use small, portable radio transceivers for communications amongst remote field locations, as well as between these remote field locations and command and control sites. Such transceivers usually operate at a relatively low power level of 20 watts or less. To achieve long distance communication capability, and to avoid line-of-sight signal transmission obstructions such as mountainous terrain, portable communication transceivers used for applications such as those described above often utilize a transponder located in an earth orbiting satellite, and are hence used in communication systems referred to as SATCOM systems.
Portable radio transceivers of the type described above must of course use an antenna to transmit and receive radio signals through space. Thus portable military where line-of-sight communications are not feasible often utilize transmissions between an orbiting earth satellite to provide the needed range and terrain obstruction avoidance. For such applications, a small manpack-transportable, SATCOM antenna operable in ultra-high frequency (UHF) radio bands is frequently used.
Manpack-transportable SATCOM antennas currently in use are required to have a reasonably high gain in UHF radio bands located generally between about 225 MHZ and 400 MHZ. Such SATCOM antennas generally have a “turnstile” type external appearance, or “form factor”, which includes a central straight, longitudinally disposed mast that has 4-radiating elements protruding radially outwards. These elements are spaced circumferentially apart at 90-degree intervals. One pair of diametrically opposed elements comprises an electric dipole antenna that is electrically connected to a first port of a hybrid antenna coupler network. The second pair of elements oriented at 90-degrees to the first pair comprises a second electric dipole antenna, and is connected to a second port of the antenna coupler network, which is shifted in phase 90 degrees from the first port by circuitry in the hybrid matching network. This arrangement results in the transmission of a circularly polarized signal. The arrangement also enables the antenna to receive at relatively high gains signals of various polarizations.
Satellite antennas of the type described above usually have reflector elements in addition to radiating elements. These typically consist of a plurality of four or more conductive rods which extend perpendicularly outwards from the antenna mast, in a plane located below the plane of the radiating elements. The reflector elements are effective in increasing the gain and directivity of the antenna both for transmitting and receiving signals.
Typical SATCOM antennas also have one or more planes of four radially disposed “parasitic” elements located in longitudinally spaced apart locations above the radiating elements. The function of the parasitic elements is to guide radio-frequency waves emitted from or received by the radiating elements to be more generally aligned with the longitudinal axis of the mast, thus increasing the gain and directivity of the antenna.
Radio communication systems using SATCOM antennas of the type described above usually are required to operate in a full duplex mode, in which signals are transmitted from and received by the antenna simultaneously. As can be readily appreciated, transmitted signals having typical power levels of tens of watts would completely overload if not destroy receiver circuitry with sufficient gain and sensitivity to process received signals having typical levels of micro watts. Therefore, full duplex operation of a SATCOM transceiver/antenna system requires an electrical isolation network to isolate transmitted, “up-link” signals in a first frequency range from received, “down-link” signals in a second frequency range. Such networks, which must be capable of handling the maximum transmitted power levels that an antenna is intended to handle, are called diplexers. Because of their power handling requirements, the diplexer circuitry must be relatively robust, and is thus usually contained in a separate box or case which is connected by flexible coaxial cables between the antenna and one or more transceivers.
The requirement for separate external diplexers imposed by prior-art portable SATCOM antennas operated in a full duplex mode places an additional burden on a soldier or other field operator, because of the additional bulk and weight of the external diplexer and cables required to interface the diplexer between a transceiver and a SATCOM antenna. Consequently, it would be desirable to eliminate the requirement for an external diplexer, The present invention was conceived in part to fulfill the need for a manpack portable SATCOM antenna operable simultaneously in different frequency bands without requiring an external diplexer.